Power modules, especially insulated-gate bipolar transistors (IGBTs), have begun to be widely used in electric power conversion devices in recent years. A power module has at least one built-in power semiconductor device and constitutes part or the entirety of a conversion connection. In addition, the power module has a structure in which the power semiconductor is electrically insulated from the base plate or the cooling surface.
An epoxy resin is used in general as a sealant for forming an electrically insulating structure. The epoxy sealing resin is suitable as a sealant because of its high dimensional stability, water resistance and chemical resistance, and electrical insulation property.
As next-generation semiconductors such as silicon carbide (SiC) have been put to practical use, semiconductor chips with higher withstand voltage have been developed. For example, conventional silicon (Si) chips have a withstand voltage of approximately 1200 V, while SiC chips have a withstand voltage ranging from 3300 V to 13 kV. A SiC chip has a high withstand voltage, and moreover is used in high temperature environments. For these reasons, a sealant using a conventional epoxy resin has problems of cracks and peeling between the resin and the metal members. In order to achieve high heat resistance, a technique is known which uses, as a sealant, a resin formed by mixing a maleimide resin being a high-heat-resistant resin and an epoxy resin. Also, a sealant is known which contains a polytetrafluoroethylene (PTFE) nanofiller in order to achieve high heat resistance and low moisture absorption.
It has become known that one cause of cracks and peeling is oxidation of a sealing resin. When a sealing resin oxidizes and deteriorates, some of the bonds in the resin polymer may be cleaved, which sometimes results in cracks and peeling in the resin and near the interface between the resin and the metal members.
Currently, companies add color to sealing resins in some cases in order to add features. In this case, molding is performed using a sealing resin which contains a coloring agent. In a power module, color fading or chalking occurs due to oxidation caused by a high temperature application test before delivery or deterioration by electrical conduction after delivery. Also, it is known that these phenomena proceed from the outer side to the inner side of the module.
In light of these circumstances, a resin in which oxidation deterioration is suppressed is demanded.